Hydrocarbons such as oil from large offshore fields can be produced by large and extremely costly production platforms that include massive rigid structures fixed to the seabed and extending up to the sea surface. However, such platforms are too costly for use in the exploitation of small offshore oil fields, particularly when the marginal field is in deep water, and may be unsuitable. A relatively low cost production system with a short lead time can be provided by utilizing a dedicated storage vessel and a transfer structure which can be connected to one end of the vessel and moored to the sea floor by a piled base or catenary chains, and which can be utilized with fluid conduits that extend from the sea floor through the transfer structure to the vessel. Such a transfer structure can include a relatively stationary portion anchored to the sea floor, and a rotatable portion which is connected to the vessel to permit the vessel to rotate without limit about the stationary portion under the influence of currents, winds, and waves. It may be noted that such a stationary transfer structure portion can move, but is restrained against movement without limit, while the vessel is able to rotate without limit about a vertical axis about the stationary structure portion.
One disadvantage in using a vessel which can drift about the transfer structure, is that rotatable connections must be made between the rotatable vessel and stationary conduits or other lines that extend down to the sea floor. One rotatable joint is a fluid swivel for carrying oil from undersea wells to the vessel. However, such wells typically produce oil at high pressures such as thousands of psi, while moderate cost fluid swivels designed to carry pumped oil normally operate at pressures of only up to a few hundred psi. Since fluid swivels are costly and high maintenance items, the cost and maintenance of the production system would be greatly increased if very high pressure fluid swivels had to be utilized to carry oil from a plurality of subsea oil wells to the vessel.
The rotational movement of the vessel relative to the fixed tower of the transfer structure, can also complicate controls for the system. A typical control arrangement utilizes many high pressure hydraulic fluid lines to operate various underwater valves and the like, with the hydraulic lines carrying pressures of perhaps a few thousand psi. While it is possible to utilize a shipboard power station and hydraulic pump and control assembly to create hydraulic control signals at thousands of psi, and to pass such signals through many fluid swivels to the stationary tower of the transfer structure, the required fluid swivel would be costly. A mooring and cargo transfer system for the transfer of fluid cargo and other fluids between a relatively stationary transfer structure portion and a rotatable vessel, which minimized the problems that can arise from rotation of the vessel, would facilitate the construction of moderate cost production systems as well as other dedicated vessel mooring systems.